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Metabolism is a key property of lake ecosystem functioning, but logistical challenges make it difficult to estimate across remote regions. The steady-state dissolved oxygen (DO) stable isotope method (18O method) estimates metabolism from discrete water samples and thus enables large-scale surveys. However, this method relies on the assumptions that the upper mixed layer DO saturation (DO%) relative to its isotopic composition (δ18ODO) is at a steady state and that an increase in DO% results in a proportional decrease in δ18ODO. The applicability of these assumptions has not been broadly assessed for small, low-productivity lakes with predominantly benthic metabolism. We evaluated the 18O method in these types of systems by surveying 184 Arctic lakes in Sweden and found that the method consistently produces realistic estimates of metabolism in well-mixed conditions and when water temperatures were relatively stable. Under such conditions, results from the 18O method agreed with those from the free-water diel DO method, and rates derived from both methods responded similarly to environmental drivers. In contrast, we found that the 18O method frequently generated unrealistic metabolic rates when temperatures were rising. Increasing temperatures may increase DO% irrespective of δ18ODO in the upper mixed layer and promote lake stratification, both violating the assumptions of the 18O method and preventing benthic metabolism from being integrated by surface water samples. We conclude that the 18O method is a powerful tool for studying metabolism in Arctic lakes across large spatial gradients, provided that temperature dynamics and vertical stratification are considered.

The stabilization and persistence of organic carbon (OC) in soils and sediments is thought to be governed in large part by association with mineral matrices, especially reactive iron (Fe) phases. However, the role of these organo-mineral interactions has not been fully evaluated in lake sediments that are typically more OC rich. Here, we conducted detailed analyses of three sediment cores collected along a lake-depth gradient from a small clearwater lake in the Swedish Arctic to explore variability in sediment geochemistry and the role of mineral protection in OC preservation. While mineralogy of the two shallower, littoral cores was similar, mostly consisting of (alumino)silicates, sediments in the deepest part also included Fe-oxides and had 6–7 times higher specific surface area, indicating a greater potential for organo-mineral interactions. Higher specific surface area was strongly associated with more thermally-labile OC (ρ = −0.71, p = 0.01), but not with OC stocks (ρ = 0.05, p = 0.89). Multiple independent proxies indicated that deep-basin OC was more aliphatic, less degraded, and likely of planktonic algal origin compared to shallower regions. We suggest that the spatial pattern in OC quality is not simply a function of different OC sources or hydrodynamic sorting, but it is also regulated by differential capacity for organo-mineral stabilization.

Land-to-water hydrological connections represent a key regulatory mechanism of carbon transport, controlling carbon dioxide (CO2) emissions from lakes; however, as of yet, there is no assessment of its role at a pan-Arctic scale across large climatic and topographical gradients. We hypothesized that hydrologically well-connected lakes in wetter regions are CO2 sources fueled by stronger lateral fluxes of external carbon relative to drier regions. However, based on data from >200 Arctic lakes, we found that lakes in drier regions have higher and more variable annual CO2 emissions (37.06.2146.0 gC m-2 yr-1, medianQ1Q3) compared to lakes in wetter regions (8.01.717.3 gC m-2 yr-1), with both the lowest and the highest fluxes recorded among dryland lakes. We hypothesize that with increasing wetness, the relative proportion of fluvial emissions increases, whereas in drier landscapes where lakes often have limited stream export, carbon inputs can be retained and more efficiently emitted from lakes.

Inland waters emit and export large quantities of carbon (C), and the balance between emission and export determines the role of inland waters in the global C cycle. Modeling studies of stream-lake networks suggest that C emission-to-export ratios (emission:export) are higher in lake-rich networks with longer network residence times (NRT), yet empirical evidence supporting these patterns remains limited. We sampled 362 stream segments and 42 lakes across 32 nested arctic stream-lake networks, repeated across four seasons with contrasting environmental conditions. The data show that lake-rich networks have much longer NRT and higher C emission:export regardless of environmental conditions, while overall C emission:export increased during warmer low-flow periods. Our findings stress the importance of accounting for lakes within stream networks and simultaneously quantifying multiple C fates when assessing the role of inland waters in the C cycle.

As climate change and nutrient pollution intensify, understanding how millions of lakes will respond to such forcings as a global or regional collective has become urgent and yet capturing their role in Earth's system remain neither conceptually unified nor empirically constrained. Here we introduce a framework that aggregates individual lake hypsography and functional attributes into composite lakes globally, across climate zones or 1-degree Earth system grid cells. We find that globally, lake shape mirrors land rather than ocean, with shallow areas dominating. This structure reveals systematic differences between glaciated and non-glaciated regions and between colder and warmer climate zones. At the 1-degree Earth system grid cells, composite lakes group into five distinct clusters. Globally, an estimated 43% of lake volume and sediment surface area lie within the mixed layer. A composite mixed layer volume-to-sediment-surface-area ratio reveals dominant water column influence and biogeochemical sensitivities, with strong contrasts across climates and glacial histories. The proposed framework advances quantifying and understanding the collective role of lakes across spatial scales in Earth's system.

Global estimates of methane (CH4) emissions from lakes to the atmosphere rely on understanding CH4 processes at the sediment-water interface (SWI). However, in the Arctic, the variability, magnitude, and environmental drivers of CH4 production and flux across the SWI are poorly understood. Here, we estimate CH4 diffusive fluxes from the sediment into the water column in 10 lakes in Arctic Scandinavia and Svalbard using porewater modeling and mass transfer estimates, which we then compare with 60 published estimates from the Arctic to the tropics. Diffusion of CH4 in the sampled lake sediments ranged from −0.46 to 3.1 mmol m−2 day−1, which is consistent with previous reports for Arctic and boreal lakes, and lower than for temperate and tropical biomes. Methane production occurs primarily within the top ∼10 cm of sediment, indicating a biogenic origin. Random forest predictive modeling of the sampled lakes revealed that conditions promoting production and deposition of autochthonous organic carbon in Arctic lakes drive CH4 diffusion into the water column by fueling sediment CH4 production. For small lakes across biomes, determinants of the estimated CH4 flux were also best captured by climate predictors, with warmer and wetter conditions favoring ecosystem productivity and enhancing flux but also lake morphometry resulting in important regional variability in estimates. Our study emphasizes the importance of quantifying diffusive CH4 fluxes from sediments in diverse lake types to account for differences in the controls on primary production and the preservation of organic carbon across and within different biomes, to refine CH4 emission estimates in a warming climate.

Dissolved organic carbon (DOC) is a key component of coastal biogeochemical cycles, but its composition and reactivity depend on the relative contribution of autochthonous aquatic versus allochthonous terrigenous DOC (tDOC). In complex coastal waters, tDOC is commonly quantified using the bulk DOC stable carbon isotope ratio (δ13CDOC). However, several limitations can hamper the use of δ13CDOC in marine ecosystems, such as (1) the narrow and often overlapping separation of the autochthonous and allochthonous endmembers, and (2) mineralization of tDOC to dissolved inorganic carbon creates a reservoir effect such that autochthonous DOC can acquire a terrigenous-like δ13CDOC. The stable isotope ratio of non-exchangeable hydrogen in the DOC (δ2Hn) has emerged as a new tool that can potentially overcome these limitations: (1) δ2Hn has a large separation between aquatic and terrigenous endmembers (>50‰) and (2) it is not subject to reservoir effects caused by tDOC mineralization. Here, we evaluate the potential of δ2Hn obtained from solid phase-extracted dissolved organic matter (SPE-DOM), by comparing it to δ13CDOC and chromophoric DOM (CDOM) optical properties. We collected samples at a site in Southeast Asia's Sunda Shelf that experiences substantial seasonal variation in tDOC input, driven primarily by the monsoon-induced physical advection of peatland-derived tDOC. Over a 1-year monthly time series, the terrigenous fraction of DOC (fterr) determined using δ2Hn of SPE-DOM and δ13CDOC of bulk DOC was well correlated (r2 = 0.42), and there was no significant difference in fterr between the two isotope systems. In fact, δ2Hn displayed slightly stronger correlations with salinity and CDOM optical properties compared to δ13CDOC. Our results indicate that δ2Hn of SPE-DOM is effective for quantifying tDOC across coastal gradients, potentially offering greater sensitivity than δ13CDOC, and is a viable alternative in settings where δ13CDOC is inadequate.

The European Standard EN 16039:2011 provides guidelines for assessing lake hydromorphology within the Water Framework Directive and includes methods for estimating volume, mean depth, and maximum depth based on statistical models applied to data from topographic maps. We tested the predictive accuracy of these models using independently collected bathymetric data from 35 Swedish lakes. The models had no predictive power, and the maximum depth predictions were inversely correlated with the observed values. The mean absolute percent error was 46% for volume and mean depth, and 85% for maximum depth. The models are flawed and should not be used.

The temperature response of ecosystem processes is key to understand and predict impacts of climate change. This is especially true for respiration, given its high temperature sensitivity and major role in the global carbon cycle. However, similar intrinsic temperature sensitivity for respiration does not mean comparable temperature effects across ecosystems and biomes because non-temperature factors can be more important. Here we analyzed soil and sediment respiration data and found that in temperature ranges corresponding to high latitude mean temperatures, absolute respiration rates aremore sensitive to non-temperature factors than to projected direct temperature effects. However, at higher temperatures (>20 °C) the direct effect of temperature mediated by temperature sensitivity will likely be more important over changes in non-temperature factors in shaping how respiration change over time. This supports past suggestions that the relatively small projected temperature increase at low (tropical) latitudes may have a large direct impact on absolute respiration. In contrast, absolute respiration rates at high (boreal/arctic) latitudes will likely bemore sensitive on the development of the non-temperature factors than on the direct effects of the large projected temperature increase there.

Social media abstract: Respirationmay be less dependent to changes in temperature at higher than lower latitudes.

The ecology and biogeochemistry of lakes in the subarctic region are particularly sensitive to changes in the abundance and optical properties of dissolved organic matter (DOM). External input of colored DOM to these lakes is an extensively researched topic, but little is known about potential reciprocal feedbacks between the optical properties of DOM and internal microbial processes in the water. We performed 28-day dark laboratory incubation trials on water from 101 subarctic tundra lakes in northern Sweden, measuring the microbial decay of DOM and the resulting dynamics in colored (CDOM) and fluorescent (FDOM) DOM components. While losses in dissolved oxygen during the incubations corresponded to a 20% decrease in mean DOM, conversely the mean CDOM and total FDOM increased by 22% and 30%, respectively. However, the patterns in microbial transformation of the DOM were not the same in all lakes. Notably, along the gradient of increasing ambient CDOM (water brownness), the lakes showed decreased microbial production of protein-like fluorescence, lowered DOM turnover rates and decreasing bacterial growth per unit of DOM. These trends indicate that browning of subarctic lakes systematically change the way that bacteria interact with the ambient DOM pool. Our study underscores that there is no unidirectional causal link between microbial processes and DOM optical properties, but rather reciprocal dependence between the two.